Background: Daily changes in ambient concentrations of particulate matter, nitrogen oxides and ozone are associated with increased cardiopulmonary morbidity and mortality, with the lungs and their function being a vulnerable target. Methods: To evaluate the association between daily changes in air pollution and lung function in healthy adults we obtained annual lung function measurements from a routine worker health surveillance program not designed for research purposes. Forced Vital Capacity (FVC), Forced Expiratory Volume in the first second (FEV1), FEV1/FVC and Peak Expiratory flow (PEF) from a cohort of 2449 employees were associated with daily measurements of PM10, NO2 and ozone at a nearby monitoring station in the North of Belgium. Repeated measures were available for the period 2011-2015. Results: The mean (SD) PM10 concentration on the day of the lung function test was 24.9 (15.5) mu g/m(3). A 10 mu g PM10/m(3) increase on the day of the clinical examination was associated with a 18.9 ml lower FVC (95% CI: -27.5 to -10.3, p < 0.0001), 12.8 ml lower FEV1 (-19.1 to -6.5; p < 0.0001), and a 51.4 ml/s lower PEF (-75.0 to -27.0; p < 0. 0001). The FEV1/FVC-ratio showed no associations. An increase of 10 aegNO2/m3 was associated with a reduction in PEF (-66.1 ml/s (-106.6 to -25.6; p < 0.001)) on the day of the examination. Conclusions: We found negative associations between daily variations in ambient air pollution and FVC, FEV1 and PEF in healthy adults.